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+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A module for working with borrowed data.
+//!
+//! # The `BorrowFrom` traits
+//!
+//! In general, there may be several ways to "borrow" a piece of data. The
+//! typical ways of borrowing a type `T` are `&T` (a shared borrow) and `&mut T`
+//! (a mutable borrow). But types like `Vec<T>` provide additional kinds of
+//! borrows: the borrowed slices `&[T]` and `&mut [T]`.
+//!
+//! When writing generic code, it is often desirable to abstract over all ways
+//! of borrowing data from a given type. That is the role of the `BorrowFrom`
+//! trait: if `T: BorrowFrom<U>`, then `&T` can be borrowed from `&U`. A given
+//! type can be borrowed as multiple different types. In particular, `Vec<T>:
+//! BorrowFrom<Vec<T>>` and `[T]: BorrowFrom<Vec<T>>`.
+//!
+//! # The `ToOwned` trait
+//!
+//! Some types make it possible to go from borrowed to owned, usually by
+//! implementing the `Clone` trait. But `Clone` works only for going from `&T`
+//! to `T`. The `ToOwned` trait generalizes `Clone` to construct owned data
+//! from any borrow of a given type.
+//!
+//! # The `Cow` (clone-on-write) type
+//!
+//! The type `Cow` is a smart pointer providing clone-on-write functionality: it
+//! can enclose and provide immutable access to borrowed data, and clone the
+//! data lazily when mutation or ownership is required. The type is designed to
+//! work with general borrowed data via the `BorrowFrom` trait.
+//!
+//! `Cow` implements both `Deref` and `DerefMut`, which means that you can call
+//! methods directly on the data it encloses. The first time a mutable reference
+//! is required, the data will be cloned (via `to_owned`) if it is not
+//! already owned.
+
+#![unstable = "recently added as part of collections reform"]
+
+use clone::Clone;
+use kinds::Sized;
+use ops::Deref;
+
+/// A trait for borrowing data.
+pub trait BorrowFrom<Sized? Owned> for Sized? {
+ /// Immutably borrow from an owned value.
+ fn borrow_from(owned: &Owned) -> &Self;
+}
+
+/// A trait for mutably borrowing data.
+pub trait BorrowFromMut<Sized? Owned> for Sized? : BorrowFrom<Owned> {
+ /// Mutably borrow from an owned value.
+ fn borrow_from_mut(owned: &mut Owned) -> &mut Self;
+}
+
+impl<Sized? T> BorrowFrom<T> for T {
+ fn borrow_from(owned: &T) -> &T { owned }
+}
+
+impl<Sized? T> BorrowFromMut<T> for T {
+ fn borrow_from_mut(owned: &mut T) -> &mut T { owned }
+}
+
+impl BorrowFrom<&'static str> for str {
+ fn borrow_from<'a>(owned: &'a &'static str) -> &'a str { &**owned }
+}
+
+/// A generalization of Clone to borrowed data.
+pub trait ToOwned<Owned> for Sized?: BorrowFrom<Owned> {
+ /// Create owned data from borrowed data, usually by copying.
+ fn to_owned(&self) -> Owned;
+}
+
+impl<T> ToOwned<T> for T where T: Clone {
+ fn to_owned(&self) -> T { self.clone() }
+}
+
+/// A clone-on-write smart pointer.
+pub enum Cow<'a, T, B: 'a> where B: ToOwned<T> {
+ /// Borrowed data.
+ Borrowed(&'a B),
+
+ /// Owned data.
+ Owned(T)
+}
+
+impl<'a, T, B> Cow<'a, T, B> where B: ToOwned<T> {
+ /// Acquire a mutable reference to the owned form of the data.
+ ///
+ /// Copies the data if it is not already owned.
+ pub fn to_mut(&mut self) -> &mut T {
+ match *self {
+ Borrowed(borrowed) => {
+ *self = Owned(borrowed.to_owned());
+ self.to_mut()
+ }
+ Owned(ref mut owned) => owned
+ }
+ }
+
+ /// Extract the owned data.
+ ///
+ /// Copies the data if it is not already owned.
+ pub fn into_owned(self) -> T {
+ match self {
+ Borrowed(borrowed) => borrowed.to_owned(),
+ Owned(owned) => owned
+ }
+ }
+}
+
+impl<'a, T, B> Deref<B> for Cow<'a, T, B> where B: ToOwned<T> {
+ fn deref(&self) -> &B {
+ match *self {
+ Borrowed(borrowed) => borrowed,
+ Owned(ref owned) => BorrowFrom::borrow_from(owned)
+ }
+ }
+}